1. Technical Field
The present invention relates to an objective lens that is used for an optical pickup, an optical disk apparatus, and the like which perform recording and/or reproducing of information on an optical recording medium such as an optical disk. The invention also relates to an optical pickup and an optical disk apparatus using the same.
2. Description of the Related Art
Recording media that have been popular for use in recording information signals include a CD (Compact Disc) using light beams of a wavelength of about 785 nm, a DVD (Digital Versatile Disc), which can be recorded with higher density than the CD, using light beams of a wavelength of about 660 nm, and a high-density-recording-capable optical disk (hereinafter, referred to as “high density recordable optical disk”), which can be recorded with higher density than a DVD, capable of recording and reproducing signals using light beams of a wavelength of about 405 nm emitted from a bluish-violet semiconductor laser. As a high density recordable optical disk, there has been proposed an optical disk, such as BD (Blu-ray Disc (registered trademark)), having a configuration in which a thickness of the cover layer (protective layer) for protecting the recording layer capable of recording signals is made to be thin.
Optical pickups have been used to record information signals on the above-mentioned optical disk such as CD, DVD, or BD or reproduce the information signal recorded on the optical disk. In the optical pickups, it is preferred that the objective lens should be made of plastic in order to reduce the entire weight and manufacturing costs. The plastic objective lens has a problem in that spherical aberration caused by temperature change is large since fluctuation in the refractive index of plastic is larger than that of glass. In particular, in a case of a high NA of the high density recordable optical disk such as BD, just several degrees change in temperature may cause a spherical aberration which greatly affects imaging performance.
As a method of correcting a spherical aberration caused by the temperature change, there has been a method of canceling the effect of the aberration by providing a diffractive structure on the surface of the objective lens and causing a spherical aberration having an opposite polarity to the spherical aberration caused by temperature change. However, in order to correct the spherical aberration caused by temperature change perfectly, it is necessary to apply a great diffractive power to the diffraction surface. Hence, it is necessary to form a very fine diffractive structure. Accordingly, there has been a problem in that it is difficult to form the diffractive structure on the lens surface and it is difficult to obtain a favorable light use efficiency because of the remainder which is cut from the mold by cutting a fine structure. Further, it has been pointed out that a cost problem also arises in that the use cycle of the mold is shortened because of complexity of the mold, yield is deteriorated by the complex diffractive structure, and so on.
On the other hand, an objective lens using the diffractive structure for compatibility of three wavelengths has been studied. It can be expected that the cost of the objective lens can be reduced greatly as compared with the general configuration having two objective lenses, but the temperature change problem also may arise when the lens is made of plastic as described above. That is, in such an objective lens, the diffraction function is used only in order to establish the compatibility of three wavelengths, and a problem arises in that it is difficult to cancel the spherical aberration caused by temperature change.
As described above, in a case where a plastic objective lens not having the diffractive structure or a so-called three-wavelength compatible objective lens is used, normally a method of correcting the above-mentioned spherical aberration caused by temperature change should be used separately. Methods of correcting the spherical aberration include a method that cancels a spherical aberration caused by temperature change by magnification of the spherical aberration by driving a collimator lens in a direction of the optical axis when there is a temperature change. Specifically, for example environmental temperature is monitored, and the collimator lens is driven in accordance with the temperature change, thereby changing the incident magnification of the objective lens. Thereby, a system that can cancel a spherical aberration caused by temperature change by causing a spherical aberration depending on a spherical magnification sensitivity of the objective lens has been adopted (refer to Japanese Unexamined Patent Application Publication No. 2008-4169).
However, on the other hand, if the incident magnification to the objective lens is changed in order to correct the spherical aberration caused by temperature change as described above, a problem arises in that the amount of comatic aberration, which is caused when the objective lens is tilted (inclined), significantly changes. Hereinafter, in order to represent the sensitivity of comatic aberration caused relative to the tilt, an amount of 3rd order comatic aberration, which is caused when the objective lens is tilted by 1 degree, is referred to as “lens tilt sensitivity”.
Some recent optical pickups are configured so that the objective lens can be tilted in a radial direction of an optical disk in order to cancel a comatic aberration caused in accordance with deformation of the optical disk and the like.
However, in a case where the lens tilt sensitivity is excessively high, signal deterioration tends to occur by slight tilt of the objective lens in accordance with changes of the incident magnification of the objective lens due to temperature change. Thus, the allowance of the lens tilt to external disturbance is lowered, and durability to perturbation is also lowered. Hence this case is not desirable. Specifically, there was a problem in that comatic aberration fluctuates significantly when the relative tilt state between the objective lens and the optical disk at the position in a tracking direction fluctuates in accordance with fluctuation in the tilt direction of the objective lens and deformation of the optical disk.
As described above, when a plastic objective lens is used, there is a problem in that the remaining or fluctuation of comatic aberration caused by fluctuation of the lens tilt sensitivity deteriorates various signals. Therefore, there is a problem in that deterioration is caused in recording and reproducing characteristics.